Most of the plastic fat products such as margarine and shortening are produced from hard stocks and liquid oils as raw materials. As an example of such a production, liquid oils such as soybean oil, corn oil, and rapeseed oil, are blended with their hardened oils (hard stocks), and the blend is adjusted to have an appropriate consistency (plasticity). The plastic fat products such as margarine and shortening thus produced tend to cause the formation of relatively coarse crystallines because fats and oils used as the raw materials are composed of fatty acids having almost the same carbon chain length, in other words, they have a highly-unified composition of fatty acids. For this reason, the plasticity of the products can be maintained at an appropriate degree only within a narrow temperature range, so that the liquid oils contained therein have a tendency to exude.
As a process for producing other hard stocks useful as a raw material of plastic fat products such as margarine and shortening, there is a well-known process using palm type oils, in which palm type oils and lauric type oils are subjected to random interesterification with a metallic catalyst such as sodium methylate (see, e.g., the specification of U.S. Pat. No. 3,949,105). According to this process, the fundamental symmetric structure of palm type oils can be modified into a random structure, and it is, therefore, possible to improve the properties of plastic fat products which will become unfavorable because the palm type oils may be gradually hardened with time when the products are being stored. However, an increase in the amount of undesirable tri-saturated triglycerides makes an inevitable rise in the melting point, and various characteristics as a plastic fat material are deteriorated by the formation of coarse crystallines; accordingly, the products may have poor characteristics of melting in a mouth. For this reason, fractionation or hardening is required after the interesterification.
If the interesterification is conducted with a lipase (see the specification of EP 0,170,431), which has, in particular, a selectivity for the 1- and 3-positions of triglycerides, it is possible to inhibit an increase in the amount of tri-saturated triglycerides. According to this process, however, palm type oils remain having the fundamental symmetric structure, i.e., having a tendency to crystallize in the .beta.-form, so that a sufficient improvement in the crystallizability as a hard stock material for use in margarine and shortening cannot be attained. This makes a problem that when they are used as a raw material of plastic food the plasticity of the food will be deteriorated during the storage.